1. Field of the Invention
This invention relates to a cross coil meter for indicating data such as a running speed of a vehicle, and more particularly to a cross coil meter in which amounts of currents supplied to cross coils are controlled by applying trains of pulses thereto.
2. Description of the Related Art
A number of measuring devices are currently available to give analog indications of values measured by sensors. For instance, measuring devices such as a speedometer and a tachometer are usually installed on an instrument panel of a vehicle so as to offer such analog indications.
The speedometer, tachometer, or the like has an indicator which swings quickly over a large extent. Cross coils are preferable drive sources for such measuring devices. The indicator is controlled by regulating amounts of currents to be supplied to the coils which cross each other at right angles.
As shown in FIG. 1 of the accompanying drawings, a conventional cross coil meter comprises two coils L.sub.s and L.sub.c which are orthogonal to each other, and a magnet M to which an indicator is fastened and is present in a magnetic field generated by the foregoing coils. The magnet M turns in response to controlled amounts of currents to the coils L.sub.s and L.sub.c. The indicator fluctuates accordingly. If a current is applied only to the coil L.sub.s, the magnet M becomes upright. Conversely, if the current is applied only to the coil L.sub.c, the magnet becomes horizontal . The magnet M leans in accordance with a ratio of the currents applied to the coils L.sub.s and L.sub.c. Further, if a direction of the currents is reversed, the magnet M changes its orientation by 180.degree.. Therefore, it is possible to turn the indicator through 360.degree. by controlling the amounts and directions of the currents applied to the coils L.sub.s and L.sub.c.
With the foregoing cross coil meter, it is required to vary the amounts of currents to be applied in a 1-100% range. The pulse width modulation (PWM) control method is preferable to such a wide range control of the currents, and is extensively utilized to control drive currents for the cross coil meter.
The PWM control method controls a duty ratio of pulses having a predetermined voltage. The amounts of currents applied to the coils are controlled assuming that the voltage is constant. Further, since the position of the magnet M (i.e. meter indication) depends upon the ratio of the current amounts, the indication remains the same even when the voltage varies. In other words, a variation of the voltage is followed by a variation of the amounts of currents applied to the coils. However, the ratio between the current amounts does not vary and the indication remains unchanged. If the currents become short, the indicator tends to extensively fluctuate due to vibration, for example. Further, the higher the voltage, the greater the currents. This means that heat is generated in the coils. To overcome this problem, there has been a strong demand to maintain the current amounts to the coils at predetermined values.
The inventor has examined conventional cross coil meters actually installed on vehicles, and has noted the following. In some cross coil meters, current amounts were gradually reduced during their use, which caused an extensive fluctuation of the indicator. It has been known that during their life time, such cross coil meters changed their temperatures, which led to variations of resistance values of the cross coils and current amounts applied thereto.